1. Field of the Invention
This invention relates to the fields of photogrammetry and stereoscopy as applied to direction and position measurements of optical targets. Images of scenes can be recorded by devices such as video cameras and digitized to yield directions of targets present in the said scenes. Data from two or more such devices whose locations and orientations are known can be combined to reconstruct the three-dimensional locations of the targets. By processing successive images recorded over time it is possible to determine the motions of the targets
2. Description of the Prior Art
Currently several technologies are available to perform direction and position measurements on optical targets by means of electronic sensors located in the image planes of measuring devices or cameras. Such devices are frequently employed to quantify motions of the human body in a variety of applications including the evaluation of patient functionality in medicine, and the enhancement of performance in athletics. Passive targets in the form of retro-reflective material or active targets in the form of light emitting diodes are attached to various body parts of interest and imaged from different locations by a number of the said devices. The application of stereoscopic principles to the target image locations measured in the image plane of each device enables the three-dimensional motions of the targets to be determined.
Devices utilizing large area tetra-lateral photo-diodes focus all light entering a lens onto a single rectangular photo-diode located in the image plane of the lens. The incident light produces currents to electrodes located at each edge of the photo-diode and the magnitudes of the currents indicate the location of the centroid of all light reaching the sensor and hence the centroid's direction relative to the device. Such devices are capable of high sampling rates but can only measure the direction of a single target at any one time. Also, these devices are subject to large measurement errors whenever light from the target is reflected or scattered by the environment and the light centroid is not the true indicator of the target's direction. Direction measurements on a plurality of targets can be accomplished by activating and measuring each target sequentially, but the technique reduces the device's effective measurement rate by a factor equal to the number of targets employed. Targets for the described device are usually constructed from light emitting diodes connected by wires to a power source controlled by a switching circuit.
Standard video systems that scan the total image focused on a Vidicon tube or a two-dimensional CCD array are routinely employed for direction measurements of optical targets. High contrast targets may be automaticaly digitized from the video signal or each frame may be totally digitized by the use of a frame grabber. Even if the total frame is not digitized the image of each target can result in considerable quantities of raw data which must be processed immediately by fast computers or stored for later processing. Additional drawbacks of standard video technology include an inadequate framing rate for many applications, and its low resolution. Non-standard video systems having higher framing rates are very expensive and provide even lower resolution and data quality. However, video technology allows the use of passive targets and is not limited to imaging a single target at one time.
Yet another target direction measuring device employs two linear direction sensors with each said sensor comprising a cylindrical convex lens system and a linear image sensor located in the image plane of the said lens system. Within each linear direction sensor the image of the target forms a line parallel to the longitudinal axis of the lens system. The longitudinal axis of the lens system will be referred to as the lens axis. The purpose of the linear image sensor is to measure the location of the target image line along a dimension orthogonal to the lens axis. A typical CCD image sensor used in such applications has high resolution, can operate at very high data rates, and comprises an elongated light sensitive area formed by a single row of discrete light sensitive elements. When used in a linear direction sensor the image sensor is mounted with the axis of its elongated light sensitive area oriented perpendiculary to the lens axis. After exposure the image sensor's light sensitive elements can be examined to determine the location of the target line and thereby establish the plane that contain both the lens axis and the target.
Two such linear image sensors having the necessary optical means may be mounted at right angles and adjacent to each other on a common plane to constitute a device for measuring the direction of a single target. If one sensor is mounted with its lens axis in a vertical orientation it determines a vertical plane containing the said lens axis and the target, and the other sensor defines another plane containing its horizontally oriented lens axis and the target. The line formed by the intersection of the two planes passes through both the device and the target and indicates the direction of the target relative to the device. The described device precludes measurement on a plurality of targets during any single exposure because N targets would result in N planes for each sensor, and these planes would intersect in N x N direction lines without providing a means to determine which intersections contain the targets. In other words, no information is available to pair the data generated by the two sensors. In order to measure a plurality of targets such systems must use multiplexed light emitting diodes with the associated inconvenience of reduced sampling rates and control wire attachments.
A related device for making three-dimensional position measurements on a single target may be constructed from three linear image sensors and the necessary optical as described by D. Mitchelson ("Techniques for the Analysis of Human Movement", Princeton Book Company, Princeton, N.J. 1976. Page 59-64) and U.S. Pat. No. 4,193,689 3/80 to Reymond and Hidalgo. This device comprises three linear direction sensors, two of the sensors having their lens axes oriented vertically and mounted at each end of a fixed bar, and a third sensor having a horizontal lens axis orientation mounted in the center of the bar. The end sensors establish two vertical planes which intersect in a vertical line containing the target and the middle sensor defines a third plane which intersects the said vertical line at the location of the target. When properly calibrated such a device yields three-dimensional position measurements of a single target. An analysis similar to that described for the two-sensor direction measurement device shows that a multiplicity of plane intersections prevents the described device from being used to carry out simultaneous measurements on a plurality of targets.
To date there has not been available a technique or device capable of making simultaneous high resolution, high sampling rate motion measurements on a plurality of optical targets.